The present invention relates to cryogenic current leads and associated methods.
The use of cryostats is well known in the art. Typically, such cryostats comprise a Dewar containing liquid helium. Such a liquid helium filled Dewar may use adiabatic demagnetization in order to realize temperatures close to absolute zero.
The extremely low temperatures of a cryostat may be used for realizing superconductivity, or otherwise investigating low temperature phenomena.
Associated with the building of many cryostats (especially adiabatic demagnetization refrigerators) is the construction of leads for conducting electrical current from a room temperature source to a large magnet or other load in the liquid helium bath. Since the thermal conductances of the cryostat itself (i.e., even without leads) and that of a lead capable of carrying 50 to 100 amps can be comparable, appropriate design of the current leads can result in the savings of several liters of liquid helium per day in order to minimize the cost of maintaining the cryostat at cryogenic temperatures.
Various previous attempts have been made to design and construct optimal current leads for cryostats. However, these previous efforts usually assume continuous use (i.e., 100% duty cycle) and tend to ignore additional parallel heat leaks to the liquid helium, the parallel heat leaks often being comparable to the heat leak of the current leads. Alternately, previous designs for low duty cycle use have employed removable leads or leads having a low conductivity section which can be slid in and out of the Dewar neck. However, these may not always be practical due to insufficient clearance over the cryostat in many cases. Additionally, and more importantly for millikelvin cryostats, heating may be introduced by the mechanical disturbance of the cryostat caused by its use. Finally, some published designs are impractical for most uses simply because of their complexity or their size (some designs take up most of the Dewar neck crossection).
Calculating the heat flow along a current lead into a helium dewar is difficult partly because the thermal and electrical conductivities .lambda.(T), .rho.(T) of metals are not simple functions of temperature, especially below 100 K. Simplifying the problem by assuming very simple forms for .lambda.(T) and .rho.(T) and ignoring aspects such as the temperature dependent viscosity of helium gas, nonideal heat exchange, and additional heat leaks have not enabled a strictly analytic solution to be found for design of a lead using only .lambda., .rho., and the cryostat dimensions. Although it is possible to design leads using numerical data for .lambda.(T) and .rho.(T), this may not be worth the trouble to an experimentalist desiring a quick means for building 100 A leads. Also, cryostat peculiarities such as thermal conductivity and gas convection patterns in the Dewar neck make such a detailed approach all the more questionable.